Many of you may remember my thread a coupld weeks ago about the metal turning gold after rinsing with water (after phosphoric acid. Some said it could be the acid turning colors, some said it is flash rust and epoxy over it wont cause problems. Some suggested to use alcohol/water to rinse it off then a good wax/grease remover.
Yesterday I blasted the other half of the car. Today I proceeded with the phosphoric acid. I then used 50% alcohol/50% water to rinse it off. After rinsing I wiped down with a towel and began to air dry it with my compressor. By the time I wiped it with the towel it had already began to yellow. In the process of air drying it got alot worse. Here are some pics, tell me what you think...... I sprayed epoxy over it anyway, same as I did the other side. Do you pros think I should strip it again and leave the acid step out of it?????
http://i128.photobucket.com/albums/p175 ... er/hhh.jpg
http://i128.photobucket.com/albums/p175 ... /mail6.jpg

Stevenf, the phosphoric acid has formed a coating of iron phosphate on the metal surface.
That's what the yellowish color is, and it's not a bad thing because it's protecting the metal
from rusting.

How strong was the p-acid solution?

I wouldn't strip it. If you're worried about adhesion, try sanding the epoxy off of a small hidden section
and see how it actually is sticking. You'll be able to tell how good the bond is between the lower
epoxy coating and the bare metal underneath......

It was mixed 3:1 as the directions stated with 3 parts water. I am not worried about adhesion because the other side did the same thing and I have already sanded that and added a second coat of epoxy. I awas just worried about the rust causeing problems in the future. So much time spent on stripping this car by hand or with a harbor freight blaster which is border line a joke... Its got me paranoid!

I wouldn't worry about rust. You've created a barrier against it with the
phosphoric acid on a molecular level, and sealed against oxygen with the
epoxy primer. No way will it rust. If the primer is sticking well
then all is good....

hello to all,

i'm a newb at this , but hope to learn as much as possible from you guys and the forum.


i have surface rust on the wheel arches of my old mercedes w124 chasis.

i have purchased a liter of a local brand rust inhibitor, which on the label lists only 2 ingredients: 1) phosphoric acid and 2) methanol.

unfortunately the rust inhibitor does not have a web site which can provide me with more detailed info. the label on the back states: "apply the rust inhibitor , and allow 2-3 hrs to dry."

I have already applied a coat of the rust inhibitor, and the next day noticed that the treated surface rust turned white, as if glazed.

have at my disposal: a can of W&G Remover, sandpaper and a liter of isopropyl

QUESTION: Should i attempt to remove the white "glaze" with the above mentioned products(w&G/sandpaper/isopropyl) prior to applying body filler, OR should i just apply the body filler over the rust treated areas?

tom

The white coating is iron phosphate. I'd do as you stated, sanding the
coating off as much as possible, cleaning with isopropyl or W&G remover,
then applying the filler.

IMO, the white coating is just dried acid, and a black coating is the covered up rust. For example, if you decide to rinse the wet acid off instead of letting it dry, then you may have faint white streaks in the panel after the rinse water dries, if it isn't rinsed thoroughly. Or if you touch the panel with your acid contaminated glove while rinsing, there will be white finger prints after the rinse water dries. Remember to keep rinsing your gloves along with the acid treated part.

But if you don't have rust pitted metal, then I wouldn't use acid, just 80 grit DA sand, clean, and epoxy primer it right after it dries. And when there is rust, I'm in the camp that likes to remove all the rust, instead of covering it up. Agitating the rust with wire brush speeds things up considerably. There are some PAINT OVER RUST products out there (which most people have failures with) but I think the metal conditioners--in auto body work--are for microscopic rust, such as a small speck in the bottom of the rust pits. But some of these products have really given metal conditioners a bad name. You wouldn't spray etch primer over visible rust, so whats the difference? JMO

Its not recommended to apply filler directly over acid treated metal (metal conditioner), or etch primer. My advice is to remove all the rust, then rinse to remove all the acid while it is completely active, then dry, DA sand, clean, dry, spray epoxy primer, then do your filler work after the epoxy dries as per your particular epoxy data sheet. ********** epoxy would be 36-48 hours to apply filler without sanding, but some epoxies cure quicker and even within the recoat window, a little scuffing would increase adhesion.

There are exceptions, such as PPG and Dupont metal conditioners, but most of the rust converters were formulated long before epoxy came along, so their directions mostly are for top coating with oil base paint (which works very well over dried rust converter). And some epoxies will also work over it, just depends on how the epoxy is formulated----so follow the paint manufacturers recommendations before applying epoxy over acid.

Ospho, is one of the most popular rust converters, and they have updated their directions to include doing a test panel to check adhesion before using epoxy over it.

What has worked best for me is to treat the rust pitted area with the phosphoric acid and then the next day apply another coat. I use dish soap and wash it off before it dries using a red Scotchbrite pad making sure to rinse it thoroughly.

Blow dry with compressed air making sure to get any residual water out of the seam and pockets where it likes to collect.

Wipe down with an automotive Wax and Grease Remover. (preferably solvent based) and when completely dry shoot your epoxy primer.

I used this method on a horse trailer frame and after a few weeks I took a chisel and tried to chip the epoxy off. I could hardly make a deep scratch on it.

There are incorrect procedures and some bad advice in this thread. The first guy from 2011 should not have used acid on blasted metal. The pores are opened up and neutralizing the acid then becomes challenging to say the least. The yellow he saw on the blasted metal was flash rust cause by the water in the blasted surface; this does not happen with such severity or at all on smooth metal. A properly neutralized panel will have a blue hue.

Epoxy should not be applied over dried phosphoric acid. The acid has to be thoroughly neutralized with water and Sotchbrite scrubbing before paint. If the acid dries it must be re-activated, just sanding the white film off is not going to fly. Ask a few epoxy manufacturers if they want their epoxy shot over poorly neutralized epoxy. ********** even states if a person has no experience they better pick up the phone and get directions for acid procedures.

I have a feeling the surface rust on that Mercedes is deeper than you think. It is probably corroded between or behind the panels/pinchwelds and is now coming through. No amount of topical treatments will stop it, it has to be cut out and repaired correctly.

ScottB wrote: The first guy from 2011 should not have used acid on blasted metal. The pores are opened up

I have a feeling the surface rust on that Mercedes is deeper than you think. It is probably corroded between or behind the panels/pinchwelds and is now coming through. No amount of topical treatments will stop it, it has to be cut out and repaired correctly.

That's what I thought also, considering the violent reaction of the metal upon contact with acid. But after leaving a blasted part in a tub of phosphoric acid for just a few minutes, it calmed down, and I think the answer is that acid works by dissolving rust and given some time it will also dissolve good metal. Think about dropping a rock in a pond of water--the displaced water rises up around the crater formed by the rock falling in. Thats what happens with blasting, its just not so pronounced, but the metal around the craters is very thin, and there is also a lot of metal dust that dissolves very quickly in acid. If you run your hand over the blasted metal after a few minutes in the acid, it will feel much smoother, and it will still have the craters but the tops will be smooth.

When trying to remove stubborn rust, it helps a lot to blast it first, then use phosphoric acid to do the last of it.

Scott, I wrote this for others because I know that you probably only use acid for etching, and I agree with what you said, if using it only for etching.
You are probably also correct about Timmm's definition of surface rust.